Organic electroluminescent devices using an organic substance are promising in applications as a cheap large-area full-color display device having a solid light-emitting solid-state device and a light source array for writing, and a number of developments have been carried out. In general, an organic electroluminescent device is constructed of a light emitting layer and a pair of counter electrodes having therebetween the light emitting layer. The light emission is a phenomenon in which when an electric field is applied between the counter electrodes, an electron is injected from the cathode; a hole is injected from the anode; the electron and the hole are re-coupled in the light emitting layer; and energy is released as light when the energy level is returned to a valence band from a conduction band.
However, in the case of such an organic organic electro-luminescent device, there is a serious problem that the luminous efficiency is very low as compared with inorganic LED devices and fluorescent tubes. Almost all of organic electroluminescent devices which are currently proposed are ones utilizing fluorescent light emission obtained by a singlet exciton of an organic light emitting material. In a simple mechanism of the quantum chemistry, in the exciton state, a ratio of the singlet exciton from which fluorescent light emission is obtainable to the triplet exciton from which phosphorescent light emission is obtainable is ⅓. Therefore, so far as the fluorescent light emission is utilized, only 25% of the exciton can be effectively applied so that the luminous efficiency is low. On the other hand, if phosphorescence obtainable from the triplet exciton can be utilized, the luminous efficiency should be able to be enhanced.
Then, in recent years, organic electroluminescent devices utilizing phosphorescence with a phenylpyridine complex of iridium have been reported (see, for example, Applied Physics Letter, 1999, Vol. 75, page 4, Japanese Journal of Applied Physics, 1999, Vol. 38, page L1502 and U.S. Pat. No. 6,303,238). These documents report that such organic electroluminescent devices exhibit the luminous efficiency of 2 to 3 times as compared with the conventional organic electroluminescent devices utilizing fluorescence.
A phosphorescent organic electroluminescent device is principally constructed of anode/hole transporting layer/light emitting layer/block layer/electron transportingting layer/cathode. The light emitting layer is principally composed of a host material for undergoing energy transfer of triplet exciton energy into a light emitting material and a phosphorescent material. In general, carbazole compounds such as CBP (4,4′-N,N″-dicarbazole-biphenyl) are frequently used as the host material.
A problem which is involved in this phosphorescent organic electroluminescent device resides in its durability, and an improvement of this problem is eagerly demanded. As a measure for improving the durability, various phosphorescent materials have been proposed (see JP-A-2001-247859). However, it is the present state that satisfactory durability has not been obtained yet.
On the other hand, there is proposed a method in which electron transporting aluminum(III) bis(2-methyl-8-quino-linato)-4-phenylphenolate (hereinafter abbreviated as “Balq2”) is used as the host material in place of the CBP (see JP-A-2003-142264).
In the foregoing document, it is designed to enhance the durability in a device using platinum porphyrin as the light emitting material and Balq2 as the host material. However, the durability is still insufficient, and an organic electroluminescent device having high luminous efficiency and excellent durability is eagerly demanded.